EP 1860755 discloses a permanent magnet rotor arrangement where a circumferential array of magnet carriers is affixed directly to the rim of a rotor body or drum. A pole piece made of permanent magnet material is located adjacent to a surface of each magnet carrier and in a channel formed in an associated inverted U-shaped pole piece retainer. The pole piece retainer is made of non-magnetic material.
Such a rotor arrangement is particularly suitable for rotors in which the flux passes through the pole pieces predominantly in the radial direction and into the rim of the rotor body. In other words, the rotor arrangement is deliberately designed so that the flux path between adjacent pole pieces flows through the body or drum of the rotor in the circumferential direction.
It has now been appreciated that for certain types of machine such a rotor arrangement can result in unacceptably high eddy current losses.